In general, there are two modes for controlling the fan rotating speed, one is voltage regulating mode and the other one is PWM mode. The voltage regulating mode is to control the fan rotating speed by utilizing a stable external voltage signal, wherein a relation between voltage and rotating speed may be showed as a curve in figure. PWM mode is to control the fan rotating speed by means of a PWM signal with a duty cycle to drive transistor switch to control the fan rotating speed, wherein the fan rotating speed will be decided depending on the duty cycle. However, if the two control modes as described above can be integrated to use, then the workable bandwidth will be increased and the problem that known driver IC is limited at a narrow bandwidth to apply can be solved practically. Referring to FIG. 1, a known fan rotating speed control circuit 10 comprises a driver IC 11, a Hall element 12, a first transistor 13, a second transistor 14 and a stator coil 15. The driver IC 11 can control the fan rotating speed only by PWM signal and the range of the workable bandwidth is relatively narrow.